Conventional heat exchangers of this type comprise a cylindrical container holding the medium to be evaporated and having a double wall forming a ring gap between an inner wall shell and an outer wall shell. The cooling medium flows through the ring gap which is equipped for an improved heat transfer between the cooling medium and the medium to be evaporated. The heat transfer improving components are ribs which may be connected to one or both wall shells.
A heat exchanger of this type is described in a publication entitled "Shuttle Orbiter Flash Evaporator", by J. R. Nason, Hamilton Standard, 79-ENAs-14, published by American Society of Mechanical Engineers, Apr. 2, 1979. In such evaporator heat exchangers, a cooling liquid that cooperates with several active cooling circuits is caused to thermally contact the medium to be evaporated. The medium to be evaporated is sprayed into the inner evaporation chamber of the heat exchanger through an injection nozzle to form a jet of liquid droplets to be evaporated. These droplets contact the inner surfaces of the double wall enclosing the evaporation chamber of the evaporator, whereby the droplets take up heat from the cooling liquid and thus are evaporated. The resulting steam or vapor is discharged into the environment of the spacecraft through an exhaust duct and port.
The operating conditions imposed by a spacecraft, especially the condition of weightlessness and the different accelerations and decelerations during starting and landing, entail a basic problem. That problem involves maintaining the medium to be evaporated and the cooling liquid of the cooling circuit in an efficient heat exchange contact with each other to assure the required high heat exchange for the evaporation. The problem becomes more pronounced because the medium to be evaporated is normally taken along at the expense of additional payload in the spacecraft. Thus, it is desirable that the medium to be evaporated is converted completely into the vapor phase or steam.
With the above problems in mind, the double wall of the evaporator container or housing is equipped with longitudinal ribs for improving the heat exchange between the cooling liquid and the medium to be evaporated. The cooling ribs extend in the flow direction of the cooling liquid in the ring gap between the inner and outer wall shell. The cooling ribs are made of corrugated sheet metal bent into a cylindrical shape connected to the inner and outer wall shells by hard soldering or brazing or the like, which are difficult time consuming operations.
The connection of the ribs with the two wall shells by soldering or welding, is quite involved and expensive in a manufacturing sense, especially in connection with modern materials, such as high strength aluminum alloys. However, such welding or brazing solves the problem that the two wall shells have a tendency to bulge in operation, especially since these wall shells are relatively thin. The bulging can be caused due to the fact that on the one hand it is desirable to keep the wall shells thin to reduce weight, and on the other hand, a hot medium flows through the ring gap between the wall shells, whereby this hot medium may even be under excess pressure. As a result, the gap facing surfaces of the wall shells forming the ring gap, are exposed to compression stress which may result at least in elastic deformations of these wall shells in a radial direction.
If the wall shells are not rigidly interconnected by the ribs brazed or welded to the wall shells, the compression stress can cause a bulging of both wall shells. Such bulging is undesirable, because it exposes the material of the heat exchanger walls to increased stress and such bulging may adversely influence the flow dynamic conditions for the cooling medium flowing through the ring gap. This problem has been encountered, especially where the ribs, different from the above described known orbiter flash evaporator, are not connected to both wall shells, but only to one of the wall shells, for example the inner wall shell while merely loosely contacting the other shell. Such a construction has substantial manufacturing advantages compared to the evaporation heat exchanger in which the ribs are brazed or welded to both wall shells. By manufacturing the ribs as an integral component with one or the other wall shell, the problems of welding inside the narrow heat exchanger gap between the two wall shells is avoided. However, the above bulging is not avoided. This problem applies, as mentioned above, especially to heat exchangers that are made of high strength aluminum alloys as is customary in spacecraft technology. Avoiding the problem in the manner taught by the above mentioned orbiter flash evaporator is involved and expensive because the ribs inside the flow gap are hard to access for the brazing or welding, whereby substantial quality control measures must be taken to assure a proper connection of the ribs to both wall shells.
U.S. Pat. No. 4,349,723 (Swiatosz) discloses a ring gap construction in which a heat transfer fin is mounted between an inner and outer wall shell by means of spot welding to hold the fin in position. The fin has a helical configuration to cause a fluid flow along a helical path through the ring gap. The spot welding merely reduces the problems without eliminating them.
U.S. Pat. No. 3,986,551 (Kilpatrick) discloses a heat exchanger structure in which parallelogram shaped fins are attached to both inwardly gap facing surfaces of the wall shells forming the gap. Neighboring fins are separated by diagonally cut grooves in such a way that one set of grooves is wider than the other set of grooves for an efficient flow control. However, no provisions are made for interconnecting the inner and outer wall shells.